1. Field of the Invention
The present invention generally relates to a mixer of a communication system, and in particular, to a mixer for use in a direct conversion receiver.
2. Description of the Related Art
Generally, a mixer or a mixer circuit (hereinafter, collectively referred to as a “mixer”) refers to a circuit for converting an input signal into a signal of a desired frequency band. The mixer is widely used in a transmitter and a receiver of a communication system and other various fields.
An example of a mixer is a mixer for use in a direct conversion receiver of a mobile communication system. The mixer mixes an input Radio Frequency (RF) signal with a signal from a Local Oscillator (LO) to output an Intermediate Frequency (IF) signal. The mixer is generally implemented with a Complementary Metal-Oxide Semiconductor (CMOS).
FIG. 1 illustrates the structure of a direct conversion receiver of a mobile communication system to which the present invention is applied.
Referring to FIG. 1, an antenna 10 receives a radio signal. A Band Pass Filter (BPF) 20 performs band pass filtering on a signal received by the antenna 10. A Low Noise Amplifier (LNA) 30 receives an RF signal that is band-pass filtered by the BPF 20 and performs low noise amplification on the received RF signal. A mixer 40 receives the RF signal that is low-noise amplified by the LNA 30 and mixes the RF signal with a LO signal applied from a LO (not shown) to generate a frequency-converted IF signal.
The mixer 40 shown in FIG. 1 can be implemented as a CMOS mixer as mentioned above. Main concerns in designing CMOS mixers are to improve conversion gain and linearity and reduce flicker noise. Flicker noise is inversely proportional to a frequency and typically occurs at a frequency less than several MHz in CMOS process. Flicker noise is also referred to as “1/f noise”. To reduce flicker noise, CMOS mixers according to prior art are suggested as follows.
Zhang, Z.; Chen, Z.; Lau, J., “A 900 MHz CMOS Balanced Harmonic Mixer for Direct Conversion Receivers,” IEEE Radio and Wireless Conference, 2000. pp. 219-222, September 2000, suggests a mixer with a static current bleeding circuitry, but it has the following disadvantages.
First, additional flicker noise occurs from the current bleeding circuitry.
Second, impedance (rds) viewed from a LO switch side increases due to very small LO switch current. Thus, some RF currents will flow into a PMOS bleeding circuitry instead of LO switching devices and conversion gain will decrease due to the diminished signal currents.
Third, some RF currents will be shunted out to the signal ground through parasitic capacitance.
Fourth, if the size of an LO switch increases high enough to reduce inherent flicker noise, parasitic capacitance will also increase and more RF currents will be shunted out to the signal ground.
Hooman Darabi, Janice Chiu, “A Noise Cancellation Technique in Active-RF CMOS mixers,” ISSCC, session 29, pp. 544-545, 2005, suggests a mixer with a dynamic current bleeding circuitry, but it has the following disadvantages.
First, a voltage at the gate of dynamic current bleeding devices (PMOS) must be high enough to turn on and off the PMOS circuit.
Second, very high LO power is required to generate a high voltage at the gate nodes of the dynamic current bleeding devices (PMOS).
Third, since conversion gain is nearly 0 dB, there is no significant difference between the mixer and an passivemixer.
Fourth, noise voltages vary in LO+ switches or LO− switches. Actually, it is impossible to dynamically inject the same amount of current to LO+ switches or LO− switches at the same time. In particular, in the implementation of an I/Q mixer, synchronization is difficult to achieve.
H. Sjoland, Ali Karimi-Sanjaani, and A. A. Abidi, “A Merged CMOS LNA and Mixer for a WCDMA Receiver,” IEEE J. Solid State Circuits, vol. 38, No. 6, pp. 1045-1050, June, 2003, suggests a mixer with one parallel connected inductor, but it has the following disadvantages.
First, due to the use of high LO currents, available headroom must be relatively narrow.
Second, the size of an inductor should be large, e.g., 10 nH, and Q of the inductor should be less than that of an inductor having a small size.
Third, it is effective to resonate tail capacitance to reduce flicker noise by an indirect mechanism instead of a direct mechanism.
G. Montagna, R. Castello et al., “A 72 mW CMOS 802.11a Direct Conversion Receiver with 3.5 dB NF and 200 kHz 1/f Noise Corner,” Symposium on VLSI Circuits Dig. October, 2004, and Sining Zhou and Mau-Chung Frank Chang, “A CMOS Passive Mixer with Low Flicker Noise for Low-Power Direct-Conversion Receiver,” IEEE J. Solid State Circuits, vol. 40, No. 5, pp. 1084-1093, May, 2005, suggest a mixer with passive implementation, but it has the following disadvantages.
First, a conversion gain is not good.
Second, an LNA gain required to minimize noise contribution from the base-band circuitry is very high.
Third, an LNA having a very high gain has a high tendency to oscillate.
Fourth, because of an LNA having a very high gain, a mixer having high linearity is required.
Fifth, since gain flatness over a frequency band is not good, the mixer is not suitable for a wideband applications.